1. Field of the Invention
The present invention relates to a pushbutton switch key sheet for use in operating portions of various electronic apparatuses, such as a mobile phone, a PDA, a car navigation apparatus, and a car audio apparatus, and more particularly, to a key sheet in which a plurality of key tops are exposed through an operational opening having no partition frame formed in a casing of an electronic apparatus.
2. Description of the Related Art
For example, for use in a mobile phone (1), there is known, as shown in FIG. 12, a pushbutton switch in which, for a reduction in size of an operating portion and an improvement in terms of an artistic design of the pushbutton switch, a plurality of key tops (4) of a key sheet (3) are exposed in a dense state through an operational opening (2a) with no partition frame, which is formed in a casing (2). The number of key tops (4) are seventeen in total, an interval between the adjacent key tops (4) is very small, for example, the interval is set to approximately 0.15 mm to 0.2 mm, and a clearance between the key tops (4) and the operational opening (2a) is also set to a very small dimension.
As shown in FIG. 13, in the conventional key sheet (3), the key tops (4) are fixed to a surface of a base sheet (5) constituting an operating surface. The base sheet (5) is equipped with base portions (6) which are formed of silicone rubber and to which the key tops (4) are fixed, and a reinforcing plate (7) formed of a hard resin. Thus, due to rigidity of the reinforcing plate (7), it is possible to suppress general distortion of the key sheet (3) if the key sheet (3) is set upright or tilted during use of the mobile phone (1). Such a conventional key sheet is disclosed, for example, in JP 2004-327417 A.
The base portions (6) are composed of pedestal portions (6a) to which the key tops (4) are fixed and flexible portions (6b) elastically supporting the pedestal portions (6a). As depressing operation is repeatedly performed on the key tops (4), there is a fear of the flexible portions (6b) suffering breakage due to bending fatigue. For example, in a portable electronic apparatus of which a reduction in thickness is urgently required, a thickness of each component has been considerably reduced, and the flexible portions (6b) are also as thin as approximately 0.2 mm, which means they are subject to breakage.
The fixation of each key top (4) to the corresponding pedestal portion (6a) is effected by applying adhesive to one of them. At this time, the adhesive squeezed out of an interface between the key top (4) and the pedestal portion (6a) may adhere to the flexible portion (6b). In the flexible portions (6b), which are very thin as stated above, portions thereof to which the adhesive adheres cease to be deformed, which leads to one-sided expansion in these areas, resulting in breakage of the flexible portions (6b).